The present invention relates to the removal of contaminants from liquids such as water and, more particularly, to the removal of such contaminants by pulsed submerged arc electrical discharges.
Biologically active molecules of diverse sorts may contaminate water and technology is sought for their removal. One example of a class of contaminants is dyes used in textile, leather, food processing, cosmetics, paper, and dye manufacturing industries. Many dyes and their breakdown products are toxic. It is desirable to remove contaminants from effluent water in manufacturing facilities before the water is discharged into the environment. Due to the aromatic rings present in the dye molecules, conventional biological treatment does not effectively eliminate these molecules.
Submerged arc discharge is an effective method of removal of biologically active molecules from liquids such as water. The submerged arc (SA) is a high current electrical discharge between two electrodes submerged in a liquid such as water. The arc produces plasma, which in turn generates ultra-violet radiation and very active oxidant species, such as (in the case of the liquid being water) OH radicals, which can destroy biologically active molecules.
It has been shown that plasma technologies can treat water. Several mechanisms including radical reactions, shock waves, ultra-violet radiation, ionic reactions, electron processes and thermal dissociation are thought to be responsible for the treatment (Locke B-R, Sato M, Sunka P, Hoffmann M-R, Chang J-S (2006) Ind Eng Chem Res 45: 882-905; Parkansky N, Alterkop B A, Boxman R L, Mamane H, Avisar D (2008) Plasma Chem Plasma Process 28:583-592; Travis J (1994) Science 264:360-362;). It is suspected that these mechanisms, singularly or synergistically, may be responsible for concurrently oxidizing trace contaminates and disinfecting. In particular, the submerged pulsed high-current and high voltage electrical discharge between two electrodes in a liquid, sometimes referred to as an electro-hydraulic discharge [Locke B-R, Sato M, Sunka P, Hoffmann M-R, Chang J-S (2006) Ind Eng Chem Res 45: 882-905], has been shown to oxidize many organic compounds such as methyl-tert-butyl ether (MTBE) [Angeloni D-M, Dickson S-E, Emelko M-B, Chang J-S (2006) Jpn J Appl Phys 45(10B):8290-8293], atrazine [Leitner N-K-V, Syoen G, Romat H, Urashima K, Chang J-S (2005) Water Res 39:4705-4714], phenol [Sun B, Sato M, Clements J-S (2000) Environ Sci Technol 34:509-513] and chlorobenzene [Liu Y, Jiang X (2008) Plasma Chem Plasma Process 28:15-24]. The electrodes were constructed from graphite, because carbon is generally biocompatible, and iron, because of the possibility of producing Fenton's reagent (H2O2, Fe2+), which very effectively oxidizes organic compounds [Tzedakis T, Savall A, Clifton M J (1989) J Appl Electrochem 19:911-921], and because any iron micro- or nano-particles which are inevitably produced can be removed magnetically [Chang J-S, Urashima K, Dickson S and Emelko M. B. S (2008) in: Guceri S, Fridman A (eds) Plasma Decontamination: Plasma Assisted Decontamination of Biological and Chemical Agents, Springer New York: 87-97]. Although electro-hydraulic discharge systems have been studied for many years, their use in water treatment applications is rather recent and remains to be optimized for various types of contaminants and microorganisms. In contrast to these high voltage systems, a low voltage arc applied between two submerged carbon electrodes was used to breakdown sulfadimethoxine in aqueous solutions [Parkansky N, Alterkop B A, Boxman R L, Mamane Avisar D (2008) Plasma Chem Plasma Process 28:583-592]. All these prior art submerged arc techniques also erode particles from the electrodes that may interfere with the process and contaminate the treated water, and should be removed.
It would be highly advantageous to have a method of removing contaminants from liquids by pulsed submerged electrical arc discharge in which the eroded particles do not interfere with the process. Ideally, the eroded particles would actually enhance the process.
There are many patents that relate to submerged arc plasma systems for purifying and sterilizing food, beverages and water.
For example, U.S. Pat. No. 6,787,105 to Robbins, titled “Process and apparatus for reduction of microorganisms in a conductive medium using low voltage pulsed electrical energy”, relates to the reduction of microorganisms in a conductive medium by subjecting the medium to low voltage pulsed electrical energy generated by pair of electrodes contacting the medium.
U.S. Pat. No. 4,917,785 to Juvan, titled “Liquid processing system involving high-energy discharge”, describes the use of a plasma arc for sterilizing, with an electrode replacement means for enabling replacement of the electrodes as they are eroded from the electrical discharge, and using a vortex to precipitate solids. U.S. Pat. No. 4,957,606 to Juvan, titled “Separation of dissolved and undissolved substances from liquids using high energy discharge initiated shock waves”, describes a system for applications including the destruction of toxic wastes and sewage treatment, precipitation of chemical compounds and elements including metals from solution (brine, sea water, industrial waste), sterilization and water purification, catalytic formation of chemical compounds, and processing of hydrocarbons.